1. Field of the Invention
The present invention relates to a television tuner in which power is supplied to an external circuit connected to, for example, a connector to which a television signal is input.
2. Description of the Related Art
FIG. 2 shows a satellite broadcast receiver for supplying power to a converter connected to an input terminal. The converter 22 converts a signal of 12 GHz received by an antenna 21 into an IF signal of 1 GHz so as to supply it to an IF input terminal 25 of a BS tuner 4 via a coaxial cable 23. The supplied IF signal is supplied to a frequency converting circuit 26 through a capacitor C to be converted into an intermediate frequency signal. The output signal of the frequency converting circuit 26 is supplied to an AGC circuit 27.
The output signal of the AGC circuit 27 is supplied to an FM demodulating circuit 28 to be FM-demodulated, thereby obtaining a video signal with an attached spread signal and 4-phase DPSK audio sub-carrier. The FM-demodulated video signal of the FM demodulating circuit 28 is supplied to a video signal processing circuit 29, and a video output signal is extracted by a de-emphasis circuit for returning a pre-emphasis video signal to its original waveform, an energy spread signal removing circuit and a video amplifying circuit to be supplied to a video signal output terminal 30.
The 4-phase DPSK audio sub-carrier is supplied to an audio processing circuit 31, wherein a 2-channel audio output signal is extracted by a 4-phase DPSK demodulator circuit for obtaining a PCM signal by demodulation and a PCM demodulator circuit for extracting the audio signal from digital signal by demodulating the PCM signal, to be supplied to the audio signal output terminals 32 and 33.
In addition, the converter 22 reduces the effect of HUM modulation since it deals with a weak signal, and, in order to reduce the size and the weight of a case body, the operation power is supplied from a BS tuner 24. The DC power supply voltage (15V) made in the power supply circuit 34 for the converter is supplied to the IF input terminal 25 via inductance L for blocking the high frequency component, and is sent to the converter 22 through the coaxial cable 23 (For example, refer to Japanese examined Patent Publication No. 06-028866(page 2, FIG. 2)
In the conventional construction, since the voltage from the power supply circuit 34 for the converter is supplied to the IF input terminal 5 via only the inductance L, there is a problem in that, when an over-voltage is applied to the IF input terminal 25 from the outside, the applied voltage is not reduced and is applied to the power supply circuit 34 of the converter to damage the semiconductor device provided therein. Also, since the power supply circuit 34 for the converter is connected to the power supply voltage supplied to the BS tuner 24, the semiconductor device used in each circuit within the frequency converting circuit 26 may be damaged by the power. Further, since the DC voltage is always output to the IF input terminal 25, a jig for performing DC-insulation by a capacitive means must be provided between the measuring instrument and the IF input terminal 25, in order to protect the measuring instrument connected to the IF input terminal 25 during manufacture and inspection of the BS tuner 24. Accordingly, the operation becomes complex. In addition, when the jig is used, the high frequency characteristics are changed, and thus it is impossible to perform accurate measurement.